Study patients, with and without histories of penicillin sensitivity, by skin testing with major and minor determinants of penicillin. The NIAID will provide major and minor determinant skin test reagents for testing purposes. The contractor will be required to adhere to a protocol to skin test patients history negative for penicillin allergy, and patients history positive for penicillin allergy with a medical indication for penicillin therapy. Treatment, withholding thereof, and/or desentization will be determined on the basis of skin test results. Studies are to be carried out on hospitalized adults (over 21 years of age) only.